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Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Income Taxes [Line Items]        
Federal statutory corporate income tax rate 21.00% 21.00% 21.00% 21.00%
Amortization of excess deferred taxes (9.00%) (9.00%) (9.00%) (8.00%)
Depreciation differences 0.00% 0.00% 0.00% 0.00%
Amortization of deferred investment tax credit 0.00% 0.00% 0.00% 0.00%
State tax 5.00% 6.00% 5.00% 6.00%
Stock-based compensation     (2.00%) (3.00%)
Other permanent items     0.00% (1.00%)
Effective income tax rate 17.00% 18.00% 15.00% 15.00%
Union Electric Company        
Income Taxes [Line Items]        
Federal statutory corporate income tax rate 21.00% 21.00% 21.00% 21.00%
Amortization of excess deferred taxes (16.00%) (12.00%) (16.00%) (12.00%)
Depreciation differences 0.00% 0.00% 0.00% 1.00%
Amortization of deferred investment tax credit (1.00%) (1.00%) (1.00%) (1.00%)
State tax 3.00% 4.00% 3.00% 4.00%
Stock-based compensation     0.00% 0.00%
Other permanent items     0.00% (1.00%)
Effective income tax rate 7.00% 12.00% 7.00% 12.00%
Ameren Illinois Company        
Income Taxes [Line Items]        
Federal statutory corporate income tax rate 21.00% 21.00% 21.00% 21.00%
Amortization of excess deferred taxes (3.00%) (4.00%) (3.00%) (4.00%)
Depreciation differences 1.00% (1.00%) 0.00% 0.00%
Amortization of deferred investment tax credit 0.00% 0.00% 0.00% 0.00%
State tax 7.00% 7.00% 7.00% 7.00%
Stock-based compensation     0.00% 0.00%
Other permanent items     0.00% 0.00%
Effective income tax rate 26.00% 23.00% 25.00% 24.00%